Structure-property relationships of rubber/silica nanocomposites via sol-gel reaction

Up to date, conventional mixing of rubber with silica is time and energy consuming process where silica aggregates are dispersed throughout the rubber matrix. A way to avoid silica aggregation and to improve dispersion is to grow silica particles directly into the rubber matrix using the sol-gel reaction. The idea triggered a PhD project where the main goal is to gain clear understanding about the structure, the formation mechanism and the corresponding properties of in-situ prepared rubber/silica nanocomposites. The gained knowledge will be then used to obtain in-situ nanocomposites via reactive extrusion process. In Chapter 1 of the thesis the current state of art in the field of nanocomposites is given, with emphasis on the preparation and properties of nanocomposites produced via sol-gel reaction. Outlined are also the objectives of the study-to provide deep understanding of the mechanism and the kinetics of the sol-gel reaction in rubber matrix and to correlate the structure of the in-situ synthesized silica to the properties of the nanocomposites. Rubber/silica nanocomposites were prepared via sol-gel reaction, using TEOS as precursor and hexylamine as catalyst. The effect of different reaction parameters - amount of the precursor, reaction time, temperature and type of the rubber on the morphology of the prepared in-situ nanocomposites (loading, size and dispersion of silica) is shown in Chapter 2. The TEM images of the nanocomposites indicated excellent dispersion of the silica particles. The amount of the bound rubber was evaluated and it was correlated to the rubber-silica interaction. The kinetics and the mechanism of the sol-gel reaction in a rubber matrix were studied in detail by performing time resolved solid-state NMR and SAXS experiments. The results presented in Chapter 3 indicate that the sol-gel process in rubber matrix adopts the emulsification process behavior, where hexylamine, used as catalyst behaves as surfactant forming inverse micelles with enclosed water in TEOS-swollen rubber matrix. The growth of the silica particles with time was probed via SAXS and a comparison is made between the growth rate of different type of rubbers and at different temperatures. The structural investigation of the in-situ synthesized silica particles, presented in Chapter 4, was performed using solid state NMR and MS-TGA-IR. For the first time we show that via the sol-gel process so called ‘hairy’ silica particles are formed, with hexylamine and remnant ethoxy groups residing predominantly on the silica surface. This "hairy" silica surface resulted in more hydrophobic nature of the silica particles, thus improved rubber-silica interactions. In Chapter 5 the properties of the in-situ nanocomposites produced in both-static and dynamic conditions (batch mixer) are discussed and compared to those of the conventionally prepared rubber/silica nanocomposites. The RPA measurements of the in-situ nanocomposites indicated strong reinforcement effect, at much lower silica loadings in comparison to the conventional rubber/silica nanocomposites. We explain this strong reinforcement in the in-situ prepared rubber/silica nanocomposites with the improved rubber-silica interactions (caused by the specific surface topology of the in-situ silica) and by the presence of trapped entanglements and bound rubber. The in-situ rubber/silica nanocomposites were successfully produced also via reactive extrusion, which was one of the targets of the thesis. As shown in Chapter 5, the obtained in this way nanocomposite had maximum loading of 3% silica, possessed uniform dispersion of the silica particles and very good properties. In Chapter 6 the effect of different silica precursors (tetramethyl orthosilicate (TMOS), (tetraethylorthosilicate) TEOS and tetrabutylorthosilicate (TBOS)) on the morphology of the in-situ nanocomposites was studied in relation to the mechanical properties as determined by DMTA and tensile testing. Chapter 7 contains the technological assessment of this study. The importance and the possibilities for industrial application of the sol-gel process to obtain rubber-silica nanocomposites with excellent properties are discussed

Structure-property relationships of rubber/silica nanocomposites via sol-gel reaction

Up to date, conventional mixing of rubber with silica is time and energy consuming process where silica aggregates are dispersed throughout the rubber matrix. A way to avoid silica aggregation and to improve dispersion is to grow silica particles directly into the rubber matrix using the sol-gel reaction. The idea triggered a PhD project where the main goal is to gain clear understanding about the structure, the formation mechanism and the corresponding properties of in-situ prepared rubber/silica nanocomposites. The gained knowledge will be then used to obtain in-situ nanocomposites via reactive extrusion process. In Chapter 1 of the thesis the current state of art in the field of nanocomposites is given, with emphasis on the preparation and properties of nanocomposites produced via sol-gel reaction. Outlined are also the objectives of the study-to provide deep understanding of the mechanism and the kinetics of the sol-gel reaction in rubber matrix and to correlate the structure of the in-situ synthesized silica to the properties of the nanocomposites. Rubber/silica nanocomposites were prepared via sol-gel reaction, using TEOS as precursor and hexylamine as catalyst. The effect of different reaction parameters - amount of the precursor, reaction time, temperature and type of the rubber on the morphology of the prepared in-situ nanocomposites (loading, size and dispersion of silica) is shown in Chapter 2. The TEM images of the nanocomposites indicated excellent dispersion of the silica particles. The amount of the bound rubber was evaluated and it was correlated to the rubber-silica interaction. The kinetics and the mechanism of the sol-gel reaction in a rubber matrix were studied in detail by performing time resolved solid-state NMR and SAXS experiments. The results presented in Chapter 3 indicate that the sol-gel process in rubber matrix adopts the emulsification process behavior, where hexylamine, used as catalyst behaves as surfactant forming inverse micelles with enclosed water in TEOS-swollen rubber matrix. The growth of the silica particles with time was probed via SAXS and a comparison is made between the growth rate of different type of rubbers and at different temperatures. The structural investigation of the in-situ synthesized silica particles, presented in Chapter 4, was performed using solid state NMR and MS-TGA-IR. For the first time we show that via the sol-gel process so called ‘hairy’ silica particles are formed, with hexylamine and remnant ethoxy groups residing predominantly on the silica surface. This "hairy" silica surface resulted in more hydrophobic nature of the silica particles, thus improved rubber-silica interactions. In Chapter 5 the properties of the in-situ nanocomposites produced in both-static and dynamic conditions (batch mixer) are discussed and compared to those of the conventionally prepared rubber/silica nanocomposites. The RPA measurements of the in-situ nanocomposites indicated strong reinforcement effect, at much lower silica loadings in comparison to the conventional rubber/silica nanocomposites. We explain this strong reinforcement in the in-situ prepared rubber/silica nanocomposites with the improved rubber-silica interactions (caused by the specific surface topology of the in-situ silica) and by the presence of trapped entanglements and bound rubber. The in-situ rubber/silica nanocomposites were successfully produced also via reactive extrusion, which was one of the targets of the thesis. As shown in Chapter 5, the obtained in this way nanocomposite had maximum loading of 3% silica, possessed uniform dispersion of the silica particles and very good properties. In Chapter 6 the effect of different silica precursors (tetramethyl orthosilicate (TMOS), (tetraethylorthosilicate) TEOS and tetrabutylorthosilicate (TBOS)) on the morphology of the in-situ nanocomposites was studied in relation to the mechanical properties as determined by DMTA and tensile testing. Chapter 7 contains the technological assessment of this study. The importance and the possibilities for industrial application of the sol-gel process to obtain rubber-silica nanocomposites with excellent properties are discussed

In situ silica-EPDM nanocomposites obtained via reactive processing

In situ rubber nanocomposites prepared via reactive batch mixing and via reactive extrusion were studied. Materials produced via reactive batch mixing showed a significantly higher silica content for a similar reaction time as compared to previously prepared in situ nanocomposites using a diffusion process under static conditions, but an approximately 10% lower modulus over the strain range investigated (0.5–1000%). While the microstructure of the nanocomposites using static conditions was homogeneous with monosized silica particles, the structure of the composites obtained via reactive batch mixing was significantly different, consisting not only of single silica particles but also of aggregates and densely packed silica regions. The nanocomposites obtained via reactive extrusion had a maximum loading of 3.2 wt% silica, possessed a uniform dispersion of silica particles and a similar modulus (˜120 kPa) as conventional nanocomposites, prepared by mixing silica and rubber, containing 10 wt% silica and a coupling agent

Influence of reaction parameters on the structure of in-situ rubber/silica nanocomposites synthesized via sol-gel reaction

In situ silica was synthesized in three non-vulcanized rubber matrices, namely natural rubber, styrene-butadiene rubber, and EPDM (ethylene-propylene diene ter-polymer), using the sol–gel method with tetra-ethoxysilane (TEOS) as silica precursor and hexylamine as catalyst. The effect of the reaction parameters such as the amount of TEOS, the reaction time (15–120 min), and the type of rubber was explored. Transmission electron microscopy was used to study the gradient in silica content and particle size over the sample thickness. The diffusion gradient of TEOS and catalyst solution in the rubber matrix responsible for the gradient was studied with Fick's law. An excellent dispersion of silica was obtained for all rubbers, even for the very non-polar EPDM, without the use of any additives to improve the dispersion

In Situ Silica Nanoparticle Formation in a Rubber Matrix Monitored via Real-Time SAXS and Solid-State NMR Spectroscopy

Silica formation in a rubber matrix is studied. The hypothesis that the formation proceeds via inverse micelles where the hexylamine present, being a catalyst, also behaves as a surfactant, is made highly plausible. As well-known, the conversion of TEOS into a solid silica particles proceeds via hydrolysis and condensation reactions. These transformations of TEOS comprise consequential substitution of the hydroxyl (-OH) groups with siloxane ones (-OSi), labeled as Q1: one -OSi group; Q2: two -OSi groups; Q3: three -OSi groups; and Q4: four -OSi groups. Here the kinetics of the sol–gel reaction in the rubber matrix was studied as a function of the reaction temperature (40–120 °C) by 1H high-resolution magic-angle spinning (HR-MAS) NMR spectroscopy. Real-time small-angle X-ray scattering (SAXS) measurements of the sol–gel reaction in NR and EPDM rubber matrices were performed in a time scale of 1 s to 60 min with images being acquired every 15 s. The sol–gel reaction in NR and EPDM rubber at 100 °C shows an initial fast increase in particle size that decreases gradually for longer reaction times, eventually reaching a plateau where the particle size does not change anymore. In the NR matrix the initial fast increase in particle size lasts approximately 25 min after which the plateau begins, while for the EPDM the initial phase is approximately 50 min. Moreover quantitative 29Si MAS NMR measurements used for mapping the silicon atoms showed no significant difference in the structure of the silica particles formed after 15 and 60 min reaction time at 100 °C, indicating that a good quality silica with a ratio Q4:Q3 ¿ 2 can be realized in a relatively short reaction time, thereby offering options for future industrial applications such as reactive extrusion processes

Chemical Mapping of Silica Prepared via Sol-Gel Reaction in Rubber Nanocomposites

Rubber–silica nanocomposites containing 10 wt % silica were prepared using in situ hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in the presence of n-hexylamine as catalyst in two rubber matrices, namely, natural rubber and ethylene-propylene diene rubber. The structure of sol–gel synthesized silica, mapped by solid-state NMR spectroscopy and XPS, indicated the presence of remnant ethoxy groups inside the silica particles and on the silica surface, while hexylamine resided preferentially at the silica surface stabilized via hydrogen bonding of the ethoxy and chemisorption of the hexylamine. Thus, the preparation of sol–gel synthesized silica results in the formation of so-called "hairy" silica particles with increased hydrophobic properties. The combinatory technique FTIR-TGA-MS confirms the complex chemistry of the sol–gel synthesized silica as well as the low amount of residual ethanol present in the particles and the in situ rubber–silica nanocomposite, the latter aspect being important when industrial manufacturing and application of in situ rubber–silica nanocomposites is considered. It is further shown that (i) the particular surface chemistry, (ii) the phenomena of entrapped rubber chains inside the silica nanoparticles, and (iii) morphology of the sol–gel synthesized silica nanoparticles lead to a more intimate interaction with the rubber matrix, which may be fine-tuned toward improved mechanical properties

Influence of reaction parameters on the structure of in-situ rubber/silica nanocomposites synthesized via sol-gel reaction

In situ silica was synthesized in three non-vulcanized rubber matrices, namely natural rubber, styrene-butadiene rubber, and EPDM (ethylene-propylene diene ter-polymer), using the sol–gel method with tetra-ethoxysilane (TEOS) as silica precursor and hexylamine as catalyst. The effect of the reaction parameters such as the amount of TEOS, the reaction time (15–120 min), and the type of rubber was explored. Transmission electron microscopy was used to study the gradient in silica content and particle size over the sample thickness. The diffusion gradient of TEOS and catalyst solution in the rubber matrix responsible for the gradient was studied with Fick's law. An excellent dispersion of silica was obtained for all rubbers, even for the very non-polar EPDM, without the use of any additives to improve the dispersion